reserve n,m for Element of NAT;
reserve h,k,r,r1,r2,x,x0,x1,x2,x3 for Real;
reserve f,f1,f2 for Function of REAL,REAL;

theorem Th23:
  [!f,x-h/2,x+h/2!] = (cdif(f,h).1.x)/h
proof
  [!f,x-h/2,x+h/2!] = (cD(f,h).x)/h by Th22
    .= (cdif(f,h).1.x)/h by Th16;
  hence thesis;
end;
